排序算法

1、归并排序

public class Main{
	public static void main(String[] args) {//归并排序
		int[] arr = {4,5,2,1,3,5,6,1,2}; 
		Main.sort(arr, 0, arr.length-1);
		for (int i = 0; i < arr.length; i++) {
			System.out.print(arr[i]);
		}
	}
	public static void sort(int[] arr,int l,int r) {//左右两部分排好顺序
		if (l==r) {
			return;
		}
		int mid = l+((r-l)>>1);
		sort(arr, l, mid);
		sort(arr, mid+1, r);	
		merge(arr, l, mid, r);
	}
	public static void merge(int[] arr,int l,int mid,int r) {
		int[] help = new int[r-l+1];
		int i = 0;
		int p1 = l;
		int p2 = mid+1;
		while (p1<=mid&&p2<=r) {
			help[i++] = arr[p1]>arr[p2]?arr[p1++]:arr[p2++]; 
		}
		while (p1<=mid) {
			help[i++] = arr[p1++]; 
		}
		while (p2<=r) {
			help[i++] = arr[p2++]; 
		}
		for (int j = 0; j < help.length; j++) {
			arr[l+j] = help[j];
		}
	}